Put llvmMathExtras in c10 namespace (#55886)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55886
We've imported llvm's MathExtras header, but now that we want to also
include LLVM (which includes its own MathExtras), we need to guard the c10
version appropriately (or interwine llvm more deeply with our build than just
the CPU fuser, which I'm not super excited about doing just yet).
ghstack-source-id: 126375067
Test Plan: build
Reviewed By: ZolotukhinM
Differential Revision: D27731038
fbshipit-source-id: 7c136341d6b433b3876ee983820016df75c14dec
